A heat roller fusing method has been generally adopted for a fuser employed in an electrophotographic apparatus, such as. a copying machine and a printer. According to the heat roller fusing method, a recording sheet having thereon formed a non-fused toner image is passed through a space between a pair of rollers heated and pressed against each other, whereby the toner image is fused and fixed onto the recording sheet. However, the heat roller fusing method has a problem that there readily occurs a so-called offset phenomenon, in which the fused toner on the recording sheet adheres to the rollers. The frequency of the offset phenomenon is particularly high with a color electrophotographic apparatus, because the color toner has poor mold release characteristics compared with the conventional black toner.
Therefore, to prevent the offset phenomenon, an offset preventing agent having small surface energy, such It as silicon oil, is essentially applied over the roller surface of the fuser employed in today's electrophotographic apparatus, especially in the color electrophotographic apparatus.
A typical oil applying device used for the fuser of this type will be explained in detail with reference to FIG. 14.
The oil applying device includes an oil applying roller 61, an oil applying felt 62, an oil limiting blade 68, an oil tank 69, etc. The oil applying felt 62 is provided in such a manner that its top end touches the oil applying roller 61, and its bottom end is dipped in oil 70 preserved in the oil tank 69. The oil applying felt 62 elevates the oil 70 in the oil tank 69 through capillarity and applies the same over the surface of the oil applying roller 61. The oil limiting blade 68 is pressed against the oil applying roller 61 at a certain pressure and scrapes off excessive oil, so that a certain amount of oil is applied uniformly over the surface of the oil applying roller 61. The oil thus leveled is transferred to a fusing roller 51 at a press contacting portion H between the oil applying roller 61 and fusing roller 51, and applied over the surface of the fusing roller 51.
As disclosed in Japanese Laid-Open Patent Application No. 111963/1983 (Tokukaisho No. 58-111963), for example, a fluorine-based porous material (for example, polytetrafluoroethylene (PTFE), such as GORE-TEX.RTM. of W. L. Gore Inc.,) is proposed as a material for the oil applying felt 62 to reduce an amount of supplied oil.
However, the conventional oil applying device has five following problems.
1 When the toner adheres to the surface of the fusing roller 51 (hereinafter, this phenomenon is referred to as offset), the toner also adheres to the oil applying felt 62 through the oil applying roller 61. When this happens, the pores of the oil applying felt 62 made of a fluorine-based porous material are clogged with the toner, and as a consequence, the oil applying felt 62 can no longer supply the oil to the oil applying roller 61.
2 An amount of applied oil over the oil applying roller 61 varies with the surface roughness of the oil applying roller 61. That is, when the surface roughness of the oil applying roller 61 is small, less amount of oil is carried beyond the edge portion of the oil limiting blade 68, and hence an amount of the applied oil over the oil applying roller 61 decreases; on the other hand, when the surface roughness of the oil applying roller 61 is large, an amount of the applied oil increases.
In the conventional oil applying device, the surface roughness of the oil applying roller 61 diminishes with the use as the oil limiting blade 68 repetitively slides over the oil applying roller 61. Thus, an amount of the applied oil is reduced eventually throughout the life of the fuser, thereby causing the offset or the like in the end. Conversely, if the oil is supplied in a sufficient amount to prevent the offset throughout the life, the oil is supplied excessively at the beginning of the life, thereby undesirably increasing an amount of the used oil.
3 An amount of the applied oil over the oil applying roller 61 also varies with a press contacting pressure at the edge portion of the oil limiting blade 68 against the oil applying roller 61. That is, the lower the press contacting pressure, the more an amount of the applied oil, and the higher the press contacting pressure, the less an amount of the applied oil.
In the conventional oil applying device, the press contacting pressure of the oil limiting blade 68 is generally higher at each end portion than at the central portion due to the flexure of the oil limiting blade 68 and oil applying roller 61 in their longitudinal directions. Consequently, an amount of the applied oil varies in the longitudinal direction. When the oil is applied nonuniformly in the above manner, there occurs an image deficiency, such as inconsistencies in gloss and unwanted transmittance variations of an OHP.
4 Since the oil applying felt 62, oil limiting blade 68 and the like are pressed against the oil applying roller 61, a driving torque of the oil applying roller 61 is so large that it has to be forced to rotate. However, it is difficult to drive the oil applying roller 61 in a stable manner, and the oil applying roller 61 readily starts to vibrate at a driving gear pitch with respect to the fusing roller 51. As a result, the oil is applied nonuniformly near the driving gear in response to the driving gear pitch (hereinafter, this phenomenon is referred to as oil banding). When the oil is applied nonuniformly in the above manner, there occurs an image deficiency, such as inconsistencies in gloss.
5 The offset occurs when an amount of the applied oil is too small. On the other hand, when too much oil is applied, an image portion remains on a recording sheet as a memory at a cycle of the fusing roller 51 (hereinafter, this phenomenon is referred to as oil ghost). The oil ghost occurs for the following reason. That is, the oil is absorbed into the toner in the image portion whereas it is not in the non-image portion, which causes considerable amount variations of the oil over the fusing roller 51. Under these conditions, when the fusing roller 51 rotates once and the oil is applied again by the oil applying roller 61, the amount variations thus caused are not eliminated and the oil is still applied nonuniformly, thereby causing the oil ghost. The oil ghost causes an image deficiency, such as inconsistencies in gloss and unwanted transmittance variations of the OHP.